By September, Texas will run out of the sole drug it uses in lethal injections thanks in part to an overseas effort that has persuaded a European pharmaceutical company to halt its supply to U.S. states for use in executions.

The Texas Department of Criminal Justice announced last week that the state’s supply of pentobarbital — the sedative used in the lethal injections of its death-row inmates — would expire in September. Pentobarbital has become the most common drug in lethal injections in the U.S. Of the 23 executions this year, 22 of them used pentobarbital by itself or in combination with other drugs.

Texas is facing a depleted supply after a Danish drugmaker announced two years ago that it would no longer supply the drug for use in executions, thanks in part to pressure from multiple groups in Europe that have unexpectedly thrown up obstacles to U.S. states carrying out the death penalty.

In early 2011, Danish drugmaker Lundbeck, which at that time manufactured pentobarbital (sold under the name Nembutal), discovered that U.S. states were using its product in lethal injections. The complex international distribution networks of pharmaceuticals often make it difficult for manufacturers to know exactly where their products end up. But once pentobarbital’s use in U.S. executions came to light, many in Denmark were upset that medicine made in a country that abolished the death penalty decades ago was being used for ending lives rather than saving them.

By spring 2011, Danish newspapers were regularly publishing stories about pentobarbital’s use as several human-rights organizations, including Amnesty International and U.K.-based Reprieve, issued press releases to highlight each new execution that used drugs made by Lundbeck. In June 2011, Dr. David Nicholl — a neurologist and human-rights activist — wrote an open letter to Ulf Wiinberg, the chief executive of Lundbeck. The letter, signed by more than 60 other doctors and academics urging the company to halt its U.S. supply, was published in the medical journal the Lancet.

“As clinicians and prescribers of Lundbeck’s products, we are appalled at the inaction of Lundbeck to prevent the supply of their drug, Nembutal [pentobarbital], for use in executions in the USA,” the letter stated. “Pentobarbital is rapidly proving to be the drug of choice for U.S. executions. Lundbeck should restrict distribution of pentobarbital to legitimate users … but not to executioners.”

Three weeks later, Lundbeck said it would no longer allow the drug to be used in U.S. executions and began reviewing all orders of the drug and denying U.S. prisons looking to order it. Now, states like Texas, Georgia and Missouri are grappling with how to continue their planned executions without their go-to drug.

“When I first approached this issue, I thought it would never work,” says Nicholl, referring to the decision to apply pressure to drugmakers supplying states carrying out executions. “But our efforts have turned out to be quite effective. I don’t think the pharmaceutical companies realized the bad p.r. that it was going to lead them to.”

To halt its supply, Lundbeck worked with human-rights group Reprieve to simplify its distribution model, essentially taking out middlemen so the company could more easily identify who ended up with its products. Maya Foa, deputy director of Reprieve’s death-penalty team, says her organization’s goal isn’t to end capital punishment in the U.S. but merely to get pharmaceutical companies to follow the Hippocratic oath to do no harm.

“Their reason to be is to make medicine to save lives,” Foa says.

The struggle to obtain pentobarbital is the latest in a series of problems that have dogged lethal injection. In 2009, Hospira Inc., a drugmaker headquartered in Lake Forest, Ill., stopped making sodium thiopental, a general anesthetic often used in a three-drug method of lethal injection. That forced many states to look overseas, but both the U.K. and the E.U. blocked their own manufacturers from supplying it to the U.S. for executions.

The obstacles to getting sodium thiopental pushed states to rely even further on pentobarbital — but now it appears that states like Texas will once again have to find another drug to take its place. There are no generic versions of the drug, and the alternatives available have yet to be either tested or used in lethal injections.

John Hurt, the director of public information for the Texas Department of Criminal Justice, says the state is considering finding another supplier of pentobarbital, a different drug altogether or possibly working with a compounding pharmacy that could create the drug specifically for the state’s executions. (Texas, which has executed 503 inmates since 1982, more than any other state by far, has two executions scheduled in September, two more in October and one in November.)

It’s unclear where Texas would find another supplier. In December 2011, Lundbeck sold the rights to pentobarbital to Illinois-based Akorn Inc. The new company, however, signed an agreement saying it would follow the same distribution restrictions as Lundbeck.

Texas could turn to a compounding pharmacy, but according to the Death Penalty Information Center, those providers don’t face oversight from the Food and Drug Administration. That often leads to questions about the drugs’ safety and its intended effects of being a more humane alternative of execution.

“Compounding pharmacies are the underbelly of the industry,” says Maurie Levin, who has represented death-sentence inmates for 20 years, referring to a sector of the pharmaceutical industry that often goes under the radar of federal and state regulators.

Hurt says the most likely scenario is that Texas will simply find another drug to replace pentobarbital, and he cites Missouri’s intention to switch to the general anesthetic propofol, which gained notoriety when an overdose of the drug was blamed for singer Michael Jackson’s death. But last year, German drug manufacturer Fresenius Kabi announced last year that it too would no longer sell the drug to states for executions, shifting its distribution with help from Reprieve.

The best hope for states like Texas is that a domestic manufacturer would agree to make drugs like propofol or pentobarbital, far from a continent that has largely done away with the death penalty. But it should be no surprise that pharmaceutical companies aren’t racing to distribute drugs that are often associated more with death than life.

Correction: An earlier version of this story stated that Fresenius Kabi is the only supplier of propofol in the U.S. Hospira and Teva Pharmaceuticals restarted manufacturing and selling the drug earlier this year.